3.4.6 Residual stress
One factor that can cause serious problems in any material is the presence of residual tensile stress. The problem often arises as a direct result of manufacturing, when hot material is shaped and then allowed to cool to ambient temperatures. For large castings like those needed to make the eye bars, such residual stress would be modified by the subsequent heat treatment to strengthen the steel, but had to be studied as part of the research effort into the catastrophic failure of the bridge.
The residual stress was investigated using several methods, including the destructive technique of removing metal layer by layer, as well as by drilling holes in the suspect sample. The surface strain was monitored by strain gauges, which indicated that there was significant stress in the eye bars near the critical inner surface next to the pin. The cuts made in order to measure the residual stress are shown in Figure 44.
The researchers reported that close to the edge of the hole in each eye bar, residual stresses were extremely high as a proportion of the total yield stress. They plotted the hoop tensile stress against the distance from the edge of the hole to produce a graph as shown in Figure 45. The upper curves show the residual stress in the inner surface of the eye bar to be of the greatest magnitude.
Such large stresses as those shown in Figure 45 point to the reason why the crack grew initially, and then, when it had reached a critical depth of about 3 mm, catastrophically. On the inner surface the largest stresses observed fall above the top measure of the graph at 160 MPa, the greatest being 190 MPa, nearly a third of the yield strength of the material. The stress tends to drop inside the bar, although in different ways; at cut 4, a compressive state is reached in the middle of the bar.
Although not sufficiently recognised at the time, residual stress in the inner edge of the eye of the bars was clearly a significant factor in the disaster. Whether or not other eye bars were examined in a similar way remains at present unknown, and the follow-up with the makers, US Steel, also unclear. The residual tensile stresses will have been formed during casting and the subsequent heat treatment, and exposed at the inner edge when the central holes were machined out. Records of the heat treatment eye bar by eye bar should have been inspected by the investigators, but whether they did see such records remains unknown. Whether US Steel knew about the problem at all also remains unknown.